1. Field of the Invention
This invention relates to positron beams and employs a radiation modifying member.
2. Description of the Prior Art
Although positrons were discovered almost fifty years ago, only recently has it become possible to use low-energy positrons as a research tool in any significant way. Partly responsible for this long hiatus has been the difficulty in obtaining sufficiently intense, well-characterized beams of positrons. If such beams could be produced they would be very useful, for instance, for solid state, surface, and plasma studies, for use in storage rings and the like in high-energy physics, and for possible devices.
Recent theoretical and experimental advances have made it possible to obtain well-characterized positron beams, which in this context essentially means quasimonoenergetic positrons. This is generally achieved by means of an efficient moderator. See for instance, A. P. Mills, Jr., Applied Physics Letters, Vol. 35, Sept. 1, 1979, pp. 427-429, where a single crystal copper moderator activated with about a 1/3 monolayer of sulfur is discussed. Briefly, positrons from a radioactive source (e.g., .sup.58 Co) or other appropriate source are caused to impinge on the activated (111) surface of a highly perfect copper single crystal. After thermalizing in the solid, some of the positrons diffuse back to the surface, where a sizable fraction of them is emitted from the solid, because the described surface has negative affinity for positrons. The emitted thermal positrons have a very small energy spread, of the order of a fraction of one eV. Typically, when a moderator of this kind is used, one obtains a slow positron flux of the order of 10.sup.-3 of the radioactive source positron activity. The area of the moderator which is emitting slow positrons is of necessity larger than the area of the radioactive source, which must have an area of several mm.sup.2 /Curie of activity to insure that the low-energy end of the positron spectrum is not self-absorbed by the source. This large spot size is a severe limitation on the achievable brightness of slow positron beams, i.e., moderated positron beams. I am using "brightness" here in its usual electron-optical sense, namely, "flux/sterad of angle subtended by the beam." Quantitatively, it is defined as follows: let the z-direction of a cartesian coordinate system be oriented along the beam direction. If N particles/unit time pass through a plane normal to z at z.sub.f, "brightness" or "luminosity" of the beam at z.sub.f is defined as ##EQU1## where .omega..sub.xy (z.sub.f) is the beam emittance, ##EQU2## with .OMEGA.(x,y) being the solid angle subtended by the particle trajectories through the point (x,y,z.sub.f), and the integral is over the plane z=z.sub.f. See for instance, P. Dahl, Introduction to Electron and Ion Optics, Academic Press, New York (1973), pp. 11-12.
Optimally achievable beam parameters can be determined from the characteristics of the beam as it is emitted from the moderator surface, since the quantity .theta.d.sqroot.E is conserved, as will be discussed in more detail below. Here .theta. is the angle of divergence of the beam, d the beam diameter, and E the beam energy. For instance, to study positron diffraction from surfaces, one typically needs .theta..about.1.degree., d.about.1 mm, and E.about.25 eV. If one starts with a moderated positron beam having E.about.0.25 eV, d.about.6 mm and .theta..about.60.degree., as occurs in a typical situation, then it is easy to see that, in order to achieve the required beam parameters, an aperture has to be employed that results in a roughly thousandfold reduction of flux.
No known method for improving the achievable brightness by means of electron optics and the like exists, since the limitation expressed in the conservation of .theta.d.sqroot.E is a fundamental one, derived from Liouville's theorem. As was indicated above, the unavailability of high-brightness, well-characterized positron beams has limited the application of positrons as research tools unitl now.